SYNOPSIS

DESCRIPTION

The PF_UNIX (also known as PF_LOCAL) socket family is used to
communicate between processes on the same machine efficiently. Unix
sockets can be either anonymous (created by socketpair(2)) or
associated with a file of type socket. Linux also supports an abstract
namespace which is independent of the file system.
Valid types are: SOCK_STREAM, for a stream-oriented socket and
SOCK_DGRAM, for a datagram-oriented socket that preserves message
boundaries (as on most Unix implementations, Unix domain datagram
sockets are always reliable and don’t reorder datagrams); and (since
kernel 2.6.4) SOCK_SEQPACKET, for a connection-oriented socket that
preserves message boundaries and delivers messages in the order that
they were sent.
Unix sockets support passing file descriptors or process credentials to
other processes using ancillary data.

ADDRESSFORMAT

A Unix address is defined as a filename in the filesystem or as a
unique string in the abstract namespace. Sockets created by
socketpair(2) are anonymous. For non-anonymous sockets the target
address can be set using connect(2). The local address can be set
using bind(2). When a socket is connected and it doesn’t already have
a local address a unique address in the abstract namespace will be
generated automatically.
#define UNIX_PATH_MAX 108
struct sockaddr_un {
sa_family_t sun_family; /* AF_UNIX */
char sun_path[UNIX_PATH_MAX]; /* pathname */
};
sun_family always contains AF_UNIX. sun_path contains the zero-
terminated pathname of the socket in the file system. If sun_path
starts with a zero byte it refers to the abstract namespace maintained
by the Unix protocol module. The socket’s address in this namespace is
given by the rest of the bytes in sun_path. Note that names in the
abstract namespace are not zero-terminated.

SOCKETOPTIONS

For historical reasons these socket options are specified with a
SOL_SOCKET type even though they are PF_UNIX specific. They can be set
with setsockopt(2) and read with getsockopt(2) by specifying SOL_SOCKET
as the socket family.
SO_PASSCRED
Enables the receiving of the credentials of the sending process
ancillary message. When this option is set and the socket is not
yet connected a unique name in the abstract namespace will be
generated automatically. Expects an integer boolean flag.

ANCILLARYMESSAGES

Ancillary data is sent and received using sendmsg(2) and recvmsg(2).
For historical reasons the ancillary message types listed below are
specified with a SOL_SOCKET type even though they are PF_UNIX specific.
To send them set the cmsg_level field of the struct cmsghdr to
SOL_SOCKET and the cmsg_type field to the type. For more information
see cmsg(3).
SCM_RIGHTS
Send or receive a set of open file descriptors from another
process. The data portion contains an integer array of the file
descriptors. The passed file descriptors behave as though they
have been created with dup(2).
SCM_CREDENTIALS
Send or receive Unix credentials. This can be used for
authentication. The credentials are passed as a structucred
ancillary message.
struct ucred {
pid_t pid; /* process ID of the sending process */
uid_t uid; /* user ID of the sending process */
gid_t gid; /* group ID of the sending process */
};
The credentials which the sender specifies are checked by the kernel.
A process with effective user ID 0 is allowed to specify values that do
not match its own. The sender must specify its own process ID (unless
it has the capability CAP_SYS_ADMIN), its user ID, effective user ID,
or saved set-user-ID (unless it has CAP_SETUID), and its group ID,
effective group ID, or saved set-group-ID (unless it has CAP_SETGID).
To receive a structucred message the SO_PASSCRED option must be
enabled on the socket.

VERSIONS

SCM_CREDENTIALS and the abstract namespace were introduced with Linux
2.2 and should not be used in portable programs. (Some BSD-derived
systems also support credential passing, but the implementation details
differ.)

NOTES

In the Linux implementation, sockets which are visible in the
filesystem honour the permissions of the directory they are in. Their
owner, group and their permissions can be changed. Creation of a new
socket will fail if the process does not have write and search
(execute) permission on the directory the socket is created in.
Connecting to the socket object requires read/write permission. This
behavior differs from many BSD-derived systems which ignore permissions
for Unix sockets. Portable programs should not rely on this feature for
security.
Binding to a socket with a filename creates a socket in the file system
that must be deleted by the caller when it is no longer needed (using
unlink(2)). The usual Unix close-behind semantics apply; the socket
can be unlinked at any time and will be finally removed from the file
system when the last reference to it is closed.
To pass file descriptors or credentials over a SOCK_STREAM, you need to
send or receive at least one byte of non-ancillary data in the same
sendmsg() or recvmsg() call.
Unix domain stream sockets do not support the notion of out-of-band
data.

ERRORS

ENOMEM Out of memory.
ECONNREFUSEDconnect(2) called with a socket object that isn’t listening.
This can happen when the remote socket does not exist or the
filename is not a socket.
EINVAL Invalid argument passed. A common cause is the missing setting
of AF_UNIX in the sun_type field of passed addresses or the
socket being in an invalid state for the applied operation.
EOPNOTSUPP
Stream operation called on non-stream oriented socket or tried
to use the out-of-band data option.
EPROTONOSUPPORT
Passed protocol is not PF_UNIX.
ESOCKTNOSUPPORT
Unknown socket type.
EPROTOTYPE
Remote socket does not match the local socket type (SOCK_DGRAM
vs. SOCK_STREAM)
EADDRINUSE
Selected local address is already taken or filesystem socket
object already exists.
EISCONNconnect(2) called on an already connected socket or a target
address was specified on a connected socket.
ENOTCONN
Socket operation needs a target address, but the socket is not
connected.
ECONNRESET
Remote socket was unexpectedly closed.
EPIPE Remote socket was closed on a stream socket. If enabled, a
SIGPIPE is sent as well. This can be avoided by passing the
MSG_NOSIGNAL flag to sendmsg(2) or recvmsg(2).
EFAULT User memory address was not valid.
EPERM The sender passed invalid credentials in the structucred.
Other errors can be generated by the generic socket layer or by the
filesystem while generating a filesystem socket object. See the
appropriate manual pages for more information.